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The Road to Responsiveness in Eclipse 3.0

The Road to Responsiveness in Eclipse 3.0. John Arthorne, Eclipse Platform Core Team. The road to responsiveness. Step 0 - If you make no changes, you will be ok (even slightly better) Step 1 - Revisit locks to reduce contention with background jobs

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The Road to Responsiveness in Eclipse 3.0

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  1. The Road to Responsiveness in Eclipse 3.0 John Arthorne, Eclipse Platform Core Team

  2. The road to responsiveness • Step 0 - If you make no changes, you will be ok (even slightly better) • Step 1 - Revisit locks to reduce contention with background jobs • Step 2 – Move long read only operations to background • Step 3 – Move long writing operations to background

  3. Step 0 – what if I do nothing? • Backwards compatibility: no worse than Eclipse 2.1 • Blockage will always be reported to the user • The UI is kept alive (painting) at all costs • Cancelation is now always possible (even during deadlock) • Lack of responsiveness in one component can kill responsiveness gains in other components

  4. Step 1 - Revisit locks to reduce contention • ISchedulingRule: generic hierarchical locks • A given scheduling rule can only be “owned” by one thread at a time • No two threads can own rules that conflict with each other • Implemented by clients – every plug-in can define their own rules • Rules can be nested to form rule hierarchies • ILock: re-entrant lock, like Java object monitors, except: • Aware of syncExec: carries over to UI thread • Deadlock reporting and recovery

  5. Step 1 – Reducing contention (continued) • IWorkspace.run was used for both batching and locking • This has been decoupled: new IWorkspace.run with scheduling rule, WorkspaceJob subclass to obtain batching inside a job • Can now use scheduling rules to lock selected resources • IResource implements ISchedulingRule

  6. Step 2 – Move read-only operations to background • Less risk in moving read-only operations into background • Can be done with little or no contention • Watch for assumptions about UI thread and thread safety • Examples: searching, indexing, decoration, repository view • We have been doing this all along, but difficulties arise • Hard to coordinate access to resources, CPU • Background processes that don’t know about each other are prone to deadlock

  7. Step 2 – Moving to the background (continued) • Job API: org.eclipse.core.runtime.jobs • Job: a unit of work scheduled to run asynchronously • Why not just java.lang.Thread? • Lighter weight: uses a shared thread pool • Support for progress feedback and cancellation • Priorities and scheduling rules • Richer scheduling: run now, run later, run repeatedly • Job listeners can find out when jobs start, finish

  8. Step 3 – Move writing operations to background • Identify the big win operations – what common tasks will the user want to be able to perform in the background? • Trade-off is added complexity of code versus important responsiveness gains • Need to be aware of concurrency requirements of code you’re calling: locks acquired, etc • Be aware of deadlock risks and know avoidance strategies

  9. Step 3 (continued) • Avoid hold and wait: • ISchedulingRule: allows jobs to specify requirements before they even start (two phase locking). • Impossible to hold a rule and be waiting for a rule • Avoid holding locks while client code is called • Avoid syncExec and use asyncExec where possible • Avoid circular wait • Always acquire locks in a consistent order • Preemption: • If all else fails, preempt the thread that introduced deadlock and report error in log. This happens for free with ILock and ISchedulingRule

  10. UI presentation of jobs • Generic job feedback • Status line animation • Progress view • User initiated jobs need strong feedback (out of box) • User wants to feel in control • When did it start/finish? • What are the results? • View-specific jobs • Tell the user if the view is busy or invalid (half-busy cursor, title font) • System jobs: no feedback at all • Must not block the user

  11. Further reading • Examples plug-in (dev.eclipse.org/home/eclipse: org.eclipse.ui.examples.job • http://dev.eclipse.org/viewcvs/index.cgi/~checkout~/platform-core-home/plan_concurrency.html • GUI Bloopers, Jeff Johnson – Chapter 7: Responsiveness Bloopers • Concurrent Programming in Java, Doug Lea • Modern Operating Systems, Andrew S. Tanenbaum

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